1. Field of the Invention
The present invention relates to a multi-beam image forming apparatus and an exposure position adjusting method, and in particular, to a technique of adjusting beam pitches of plural light beams in a main-scanning direction.
2. Description of the Related Art
Conventionally, in response to a demand for an increase in the speed of image output, there has been proposed an image forming apparatus including a multi-beam exposure section that scans plural light beams over a photoreceptor at the same time. In order to obtain a high-quality image using such an image forming apparatus, it is important to appropriately adjust beam pitches (intervals) of the plural light beams scanned over the photoreceptor in the main-scanning direction and the sub-scanning direction.
In general, in the case of the image forming apparatus, rough adjustment (without outputting a pattern image for evaluation) is performed for the multi-beam exposure section alone. Thereafter, the multi-beam exposure section is assembled to the actual apparatus, and pattern images for evaluation are output by varying exposure-start timings for each of the light sources. Then, an adjustment operator visually evaluates the pattern images for evaluation to make fine adjustment (adjustment involving effects of the photoreceptor or other units).
For example, as for adjustment of the beam pitches in the sub-scanning direction, there is disclosed a technique of outputting an image pattern for evaluation to detect a slight change in beam pitches of plural light beams in the sub-scanning direction (see, for example, Patent Literature 1).
Furthermore, there is disclosed an evaluation chart for easily checking a positional shift of irradiation of plural light beams in the sub-scanning direction (see, for example, Patent Literature 2).
Moreover, there is disclosed an evaluation chart with which displacement of light beam pitches in the sub-scanning direction can be detected in a highly precise manner (see, for example, Patent Literature 3). The evaluation chart includes an image evaluation pattern in which plural image patterns formed by a combination of plural different light beams are arranged so as to be adjacent to each other in the main-scanning direction, the image pattern being formed by repeating n dot lines (n≧2), which are formed in the main-scanning direction, in the sub-scanning direction at a period of an integral multiple of the number of light beams.